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A thin but noticeable haze settled over the Sierra foothills Monday, causing some Nevada County residents to fret about the possibility of a nearby wildland fire.

No new fires were detected in the immediate vicinity as the discernible layer of smoke traveled from an unusual place, said Karl Swanberg, forecaster for the National Weather Service.

The Douglas Complex fire burning in southern Oregon created a large plume, which drifted southeast over the West Coast and Pacific Ocean before being picked up by the westerly Delta breeze and deposited in the Central Valley and Sierra foothills.

“The Oregon fires are especially heavy smoke producers, and in a somewhat unusual pattern, the smoke has been traveling offshore, then south over the ocean and back inland in the San Francisco Bay Area,” states a release issued by the Nevada, Plumas and Sierra County Air District.

Additionally, particulate matter deriving from the Aspen Fire, an 11,000-acre wildland fire burning in the Sierra Nevada near Fresno, just south of Yosemite National Park, has drifted north.

The confluence of the exhaust from the two conflagrations has created an air quality health advisory, according to the air district. The pattern is expected to continue through today and possibly beyond.

Smoke concentrations are expected to intermittently be in the “Unhealthy for Sensitive Groups” range throughout the region and are expected to vary during the course of each day depending on wind speed, wind direction and other smoke dispersion factors, the release states.

Smoke may settle in low-lying areas overnight and is likely to be spread across the northern Sierra throughout the daytime hours.